Generally, sections of rectangular ducts are connected by transverse joints, one of which uses the recently developed slip-on type of rollformed flange.
In the North American sheet metal industry, a regulatory body called "Sheet Metal and Air Conditioning Contractors National Association, Inc." (SMACNA) publishes construction standards for round and rectangular ductwork. The standards in rectangular ducts require that all ducts be constructed within the following limitations:
(i) Maximum transverse joint or transverse reinforcement deflection must not exceed 0.250"; PA0 (ii) Maximum combined joint and centre sheet deflection must not exceed 0.750"; PA0 (iii) The maximum allowable design stress is 24,000 psi. PA0 (i) F class joint or reinforcement can be spaced a maximum of 5 ft using 16 gauge duct material. (The classes of joints or reinforcements have different stiffnesses). PA0 (ii) F class @5 ft using 18 ga. PA0 (iii) F class @4 ft using 20 ga. PA0 (iv) E class @3 ft using 22 ga. PA0 (v) E class @21/2 ft using 24 ga. PA0 a third layer of malleable sheet material overlying the first and second layers, with the second layer sandwiched between the first and third layers, PA0 the first layer defining a first displaced pocket projecting away from the second layer, PA0 the second layer defining a second displaced pocket nested within said first displaced pocket, PA0 the third layer having an aperture in registry with both pockets, the aperture resulting from the punching out of PA0 a disc which is nested within the second displaced pocket. PA0 (a) a base leg defined by two juxtaposed layers of sheet metal, and PA0 (b) an upstanding leg defined by at least one layer of sheet metal, PA0 (c) all said layers being integral with each other, PA0 the stiffener being juxtaposed with its base leg against said flat wall so that there are defined first, second and third layers of juxtaposed sheet metal, the second layers being sandwiched between the first and third layers, PA0 the stiffener being joined to the said flat wall by a plurality of mechanical connections, at each connection: PA0 (d) the first layer defining a first displaced pocket projecting away from the second layer, the first pocket having integral bottom and side walls, PA0 (e) the second layer defining a second displaced pocket nested within said first displaced pocket, the second pocket having integral bottom and side walls, PA0 (f) the third layer having an aperture in registry with both pockets, the aperture resulting from the punching out of PA0 (g) a disc which is nested within the second displaced pocket.
In order for SMACNA to accommodate limitations of most fabricating shops and to incorporate the limitations set out in the construction codes, it was necessary for SMACNA to prepare suggested construction alternatives as a guide for the sheet metal contractor.
As an example, consider a pressure of 4" W.G. (inches of water gauge: 1" W.G.=0.0361 psi), and a duct ranging in width from 37" to 42". For this situation there are five alternative constructions:
Most contractors operate with either a 4 ft or 5 ft line. This means that their most cost-effective joint spacing would be 4 ft or 5 ft, depending on the size of line they have. The size of line reflects the width of sheet or coil which is used.
Consider a contractor who stocks 4 ft material, either 4.times.8 ft sheets or 48" coils. If this contractor is required to construct 3 ft joint spacings, he will have considerable scrap losses. Ideally, the contractor wishes to construct his duct from one joint spacing.
In order to maintain his spacing and still satisfy the SMACNA construction requirements, he must change gauges. Most contractors store 4 or 5 gauges in the shop to cover the majority of the work. This means that the contractor is buying smaller quantities per gauge, and usually paying a premium. He buys either from a service centre or from the steel mills directly. In either case, small quantity purchases usually mean paying a premium for the sheet metal.
Furthermore, if a contractor with a 4 foot line had a choice of constructing a duct from 20 gauge joint spaced at 4 ft or a duct from 24 gauge joint spaced at 4 ft plus an intermediate reinforcement at 2 ft, he usually would build the most economical one, which is the heavier gauge material. This follows because of the very high labour rates and the amount of material and labour involved in building the intermediate reinforcement. For this reason, it is usually cheaper to install the heavier gauge alternative without the reinforcement at 2 ft spacing.